1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to an access method between an access point and one or more stations of the wireless communication system.
2. Description of the Prior Art
Many wired and wireless networks enable multiple access using distributed channel access mechanisms. In a typical instantiation, such as that used in the IEEE 802.11 set of protocols, devices that share access to the wireless medium, and that wish to transmit, will independently choose a number of transmission-available time slots (a ‘backoff’) to delay before attempting to transmit a packet. A new backoff is chosen for each new packet. Each device's new backoff is randomly (or pseudo-randomly) chosen from a set of possible backoffs determined by the protocol. Different devices will (usually) choose different backoffs (because the backoff choices are made independently on each transmitter), and then the devices will not attempt to transmit at the same time. In this way the protocol allows for different devices that implement the same underlying protocol to transmit at different times, without any explicit sequence of negotiation or arbitration messages. This is a “distributed” coordination channel access mechanism.
However, two or more devices may choose the same backoff value. In this case a “collision” will occur.
For this distributed coordination system to work effectively, the number of available backoff slots must be appreciably greater than the number of different devices actively seeking to transmit at a given time. First, unless there are as many available backoff slots as devices actively seeking to transmit, it will be impossible for each device to choose a different backoff slot. Secondly, if the number of devices actively seeking to transmit is large, and the number of backoff slots equals only slightly exceeds the number of devices, it will be highly improbable for a satisfactory outcome to occur.
As one example, if the same K available backoff slots are available for each of N devices actively seeking to transmit, then the probability that the backoff slot that is chosen by a given device is not chosen by any of the other devices is (1−1/K)N-1. If K=r*N, and N is large, this is approximately e−1/r. For this probability to exceed 0.9 (so that the probability that this device will be involved in a collision in its next transmission will be lower than 0.1), r should be at least 9.5: that is, there should be at least 9.5 times as many available backoff slots as devices actively seeking to transmit. For this probability to exceed 0.95 (so that the probability that this device will be involved in a collision in its next transmission will be lower than 0.05), r should be at least 19.5. This in turn causes many backoff slots to go unused, lowering overall network throughput.
Since the loss of overall network throughput due to a collision (loss of use of the shared medium for the duration of the longest packet that any of the colliding devices transmits) usually exceeds the loss due a backoff slot going unused (loss of use of the shared medium for the duration of the backoff slot), it is generally desirable to make the probability of a collision correspondingly lower, requiring an increased ratio r of available backoff slots to number of devices actively seeking to transmit, and lowering overall network throughput.
Alternatively, a common procedure used in wireless systems is for the transmitter to send an initial short packet announcing the proposed/desired duration of the current frame exchange sequence and for the receiver to respond declaring the wireless medium apparently free for that duration, followed by the actual data transmission. In this way, a collision between two or more transmitters employing this strategy will only cause a futilely busy wireless medium for the duration of the initial short packet, rather than the (possibly much longer) duration of the intended data packet. In the IEEE 802.11 protocol, the initial short packet is a “Request-to-Send” (“RTS”) packet. This common strategy mitigates some of the negative effects of a collision, but at the cost of additional protocol overhead, since where there is no collision the protocol requires the additional full exchange (in IEEE 802.11, an RTS, followed by a short inter-frame space (“SIFS”), followed by a “Clear-to-Send” (“CTS”), followed by another SIFS), which comprises significant non-data-carrying overhead, which it is desirable to avoid. The method of the present invention may additional be used, without limitation, to comprise this and other sources of protocol overhead that arise from the possibility of collisions in the baseline CSMA/CA (carrier sense multiple access with collision avoidance) system.